Caption: Methionine. Molecular model of the essential amino acid methionine (C5.H11.N.O2.S). This sulphur-containing alpha-amino acid is one of the amino acids that is a precursor to proteins (proteinogenic). Atoms are represented as spheres and are colour-coded: carbon (grey), hydrogen (white), nitrogen (blue), oxygen (red) and sulphur (yellow). Illustration.